Preparation of granular aggregate for use in concrete objects and structures

ABSTRACT

The invention relates to a process of preparing a lightweight granular aggregate for use in concrete objects and structures, by an intimately mixing a clayey material with materials that help to reduce the weight, granulating this mixture and then heating the granules at least to the temperature at which their mass expands. Use is made of a combination of two known auxiliary materials, the first being a porous inorganic material with open pores, added in an amount of 10-20 percent by volume with respect to the clayey material, and substantially having a grain size below 2 mm., and the second consisting of one or more substances that, upon heating, yield gaseous components at a temperature over 800* C. Use is made of a porous inorganic material having a softening temperature equal to or lower than that of the clayed material.

Lemmens Apr. 2, 1974 PREPARATION OF GRANULAR AGGREGATE FOR USE INCONCRETE OBJECTS AND STRUCTURES Inventor: Jan C. Lemmens, Beek (L),

Netherlands Assignee: Stamicarbon N.V., Heerlan,

Netherlands Filed: Feb. 25, 1972 Appl. No.: 229,587

Related US. Application Data [63] Continuation of Ser. No. 40,383, May25, 1970,

abandoned.

[52] US. Cl. 106/288 B, 106/40 R [51] Int. Cl. C08h 17/02 [58] Field ofSearch 106/40 R, 67, 68, 288 B [56] References Cited UNITED STATESPATENTS 2,670,298 2/1954 Froksaer-Sensen et al 106/40 2,699,409 l/l955Hashimoto 117/100 2,706,844 4/1955 Nicholson 25/156 PrimaryExaminer-Delbert E. Gantz Assistant ExaminerS. Berger Attorney, Agent,or Firm-Cushman, Darby & Cushman [57] ABSTRACT The invention relates toa process of preparing a lightweight granular aggregate for use inconcrete objects and structures, by an intimately mixing a clayeymaterial with materials that help to reduce the weight, granulating thismixture and then heating the granules at least to the temperature atwhich their mass expands. Use is made of a combination of two knownauxiliary materials, the first being a porous inorganic material withopen pores, added in an amount of 10-20 percent by volume with respectto the clayey material, and substantially having a grain size below 2mm., and the second consisting of one or more substances that, uponheating, yield gaseous components at a temperature over 800 C. Use ismade of a porous inorganic material having a softening temperature equalto or lower than that of the clayed material.

2 Claims, N0 Drawings This is a continuation, of application Ser. No.40,383, filed May 25, 1970, now abandoned.

The present invention relates to a process of preparing a granularaggregate for use in light-weight concrete objects and structures, byintimately mixing a clayey material with materials that help to reducethe weight, ,granulating this mixture, and then heating the granules atleast to the temperature at which their mass expands. The term Clayeymaterial as used here denotes a material at least half of which consistsof clay minerals, the non-clayey component being constituted mainly byminute quartz particles. Clayey material also includes fine-grainedmaterials obtained from shale or slate by grinding or formed therefromas a result of weathering.

As is well known, expanded, and hence lightweight, granules consistingof clayey materials can be used 'to replace gravel as an aggregate inhigh-quality building concrete. If the concrete should be light inweight, granules of the lowest possible weight-to-volume ratio areapplied: these granules must be sufficiently strong and show goodadherence to the mortar, if the concrete is to have the desiredstrength. The granules should not absorb too much water, if the materialis to be transported from a stationary concrete mixing plant (premixedconcrete); this property also ensures that the concrete will soon reacha high compressive strength, which is a necessary requirement forprefabricated units of prestressed concrete.

For granules prepared from clayey materials there is a rough correlationbetween their volume-to-weight ratio on the one hand, and theirstrength, as well as the ultimate compressive strength of the concretemade with them, on the other. To reach a compressive strength of about500 kg/cm after 28 days setting and hardening, concrete cubes having anedge of cm and made of concrete showing the composition normal forbuilding concrete, the granules used must have a volume-to-weight ratioof 1.2 1.5; see, for instance, Beton, Herstellung und Verwendung,Beton-verlag, Duesseldorf, 16 (1966), 3, p. 4, FIG. 3. Granules of lowerspecific gravity have so far failed to effect such a compressivestrength, at least if they were made of clayey materials.

After formation of the granules, the further prepara tion is carried outin a known way, by drying the granules and then heating them to thesoftening temperature of the clay, which results in a vitreous denseskin being formed around the granules. At the same time, gases cause thematerial then plastic to expand, so that it becomes porous. The size ofthe cavities in the porous structure, and the thickness of the wallsbetween the cavities, ultimately determine the volume-toweight ratio andthe strength of the granules.

Another known way of preparing granules of low volume-to-weight ratio ismixing the clay, before formation of the granules, with organic orinorganic substances capable of creating cavities. Examples of suchsubstances are foam plastics, sawdust, and carbon. The US. Pat. No.2,699,409 mentions perlite as an aggregate. The addition of organicsubstances has the drawback that carbon is introduced into the clay,which results in an appreciable drop in strength, whereas addition ofinorganic substances has the disadvantage that,

to prepare light granules, such a quantity of relatively large particleshas to be added that industrial-scale formation of granules from such amixture becomes difficult.

Further, the Netherlands Patent Specification No. 91,469 describes aprocess for the preparation of a lightweight concrete aggregate from amixture of coarse-grained shale, fine-grained shale, and relativelycoarse-grained, ignited and porous shale. Heating to a sufficiently hightemperature will then result in granules being formed from this materialby sintering. The

.granules, consisting of the sintered mixture, have open pores and donot possess a dense skin, so that they can absorb much water, which isless desirable if a satisfactory concrete mix is to be made.

Finally, from Concrete Technology and Practice (1965), p. 462 and 463,as also from the US. Pat. No. 264,137, it is known that auxiliarymaterials can be added which upon heating yield gaseous componentspromoting the expansion of the clay heated to the softening point. Ifthese materials are used, the cavities formed in the softened clay massgenerally become too large.

The invention provides a process of preparing a granular aggregate foruse in light-weight concrete objects and structures with avoidance ofthe abovementioned drawbacks and difficulties. According to theinvention, use is made of a combination of two known auxiliarymaterials, the first being a porous inorganic material with open pores,added in an amount of 10-20 percent by volume with respect to the clayeymaterial, and substantially having a grain size below 2 mm, and thesecond consisting of one or more substances that, upon heating, yieldgaseous components at a temperature of over 800C. This process makes itpossible to prepare a granular aggregate suitable for making a concretewith a compressive strength of at least 300 kg/cm and a volume-to-weightratio of the granules of about 1.0 or even lower. The material obtainedis very suitable for use in pre-stressed or other light-weight concretestructures.

The porous material forms nuclei for the expansion of the gases evolvedby the second auxiliary at high temperature, which nuclei arehomogeneously distributed throughout the clay. It is, consequently,possible to control the strength and the volume-to-weight ratio of thegranules at will, through a proper choice of the number and size of thecavities in the granules and of the related thickness of the wallsbetween the cavities. The number and the size of the cavities depend onthe amount and the size of the porous inorganic material added, and onthe amount of the second auxiliary(ies), which is adapted to that of thefirst. Thanks to the controlled structure, with homogeneous distributionof well-defined expansion cavities, it is further possible to obtain astronger and denser skin than has so far been the case. Since, moreover,the porosity of the material causes some roughness of the granulesurfaces, the granules will in general adhere better to the mortar thandoes gravel. The amount of porous inorganic material to be mixed withthe clay is so low, and its grain size so small, that further processingto shaped granules does not meet with any difficulty.

As will appear from the examples and this is an important advantage ofthe invention the effect, as regards the volume-to-weight ratio, isbetter than could be expected with addition of either of the auxiliarymaterials alone, and a given strength is obtained at a lower ratio. Inthe process of the invention, the process conditions are much lesscritical than in the known processes, since no demands are made on thenatural expansion of the clay, and the size and number of the cav- 5ities are not left to chance.

In carrying out the process of the invention a nonexpanding or weaklyexpanding clay may be used, although there is no need to do this. Bypreference use is made of a clay having a relatively high softeningpoint and a high melting point, e.g. 1,350C, and 1,600C, respectively.However, it is also possible to start from weathered and/or ground slateor shale, the latter being the slaty refuse separated from the impurerun-of-mine coal in a coal washery.

By preference use is made of a porous inorganic material having asoftening temperature equal to or lower than that of the clayeymaterial. This enables the inorganic material ultimately to deposit as ahard coating on the walls of the expansion cavities. Materials verysuitable for the purpose are pumice and similar natural materials.Manufactured materials of the same kind can be used too, however.Further, owing to the presence of a porous inorganic material during thepreparation, oxygen can easily penetrate to the inside of the granulesformed. By heating in an oxidizing medium at a temperature over 550C,preferably in the range of 550C 850C, any carbon in the clayey material,the presence of which is undesirable, may be removed fairly rapidly, sothat the granules of the end product are white, light grey, or possibly,red inside, unlike granules prepared by known processes, which areinvariably black on the inside. Even FeO, which produces a blackcolouration of the inside, can easily be con- 35 verted into an oxide oflighter colour.

Particularly suitable for use as the second auxiliary material is amaterial giving off gaseous components at a temperature close below themelting point of the porous silicate, for instance calcium sulphate.Other sulphates, sulphides, carbonates and the like, or combinationsthereof, can also be used. The weight percentage of the second auxiliarymaterial(s) in relation to the basic material need not be more than 5.

EXAMPLE 1 This example relates to the preparation of an aggregate from aclay of the following analysis:

The porous material used was a white pumice, of the following analysis:0

softening point 1,200 "C melting point 1,360 "C grain size: 96 between 1mm and A mm.

Of this clay, spherical granules of about 8 g were formed, with thefollowing composfion A: clay only B: 97.5 wt of clay C: wt of clay D:92.5 wt of clay 2.5 wt of gypsum 5 wt of pumice 5 wt of pumice, 2.5 wtof gypsum.

Composition Volume-to- Permissible load Colour on inside weight ratioafter heating to 1380 C A 1.35 1 10 black B 1.25 129 dark grey C 1.25122 light grey D 0.90 125 white water absorption after 30 minutes: 3%

The above figures show that addition of 5 percent by weight of pumice(composition C) does not suffice to effect an appreciable drop of thevolume-to-weight ratio as compared with that of the clay-only granules.Neither does the use of gypsum alone (composition B) have a pronouncedeffect. However, combined addition of pumice and gypsum (composition D)to the clay results in an appreciable lighter granule of virtually thesame strength.

With the use of these granules a structural concrete was composedcontaining 375 kg of Portland-B cement per cum and having a slump ofabout 8 cm. After 28 days the compressive strength of a cube having anedge of 10 cm was 595 kgf/cm The weight by volume of the concrete was1.68 kg/litre. The compressive strength of a 20 cm cube made from aconcrete prepared with 375 kg of Portland B cement per cum and normalriver gravel of exactly the same size distribution as the above granulesamounted to 536 kgflcm and the weight by volume was 2.39 kg/litre. Thematerial obtained by the process of the invention is highly suitable foruse in prestressed concrete structures.

EXAMPLE II In a semi-technical experiment use was made of the same clayas employed in example I, to which were added 7.5 percent wt of pumice(so-called bims) with a size range of 1%mm, and 2.5 percent wt ofgypsum. After formation, on a rotary pelleting table, of granules ofsize fractions 3-7 mm and 7-15 mm, these granules were dried on a chaingrate and slowly preheated in an oxidizing medium to a maximumtemperature of 800C. The preheated granules were then transferred to arotary kiln and there heated to 1,370C, after which they were rapidlycooled.

The resulting material, which was white inside, was tested in a buildingstructure, the concrete for which had the following composition:

35 kg of Portland B cement 572 kg of river sand 48 kg of fine-grainedsand 250 kg of 3-7 mm granules 400 kg of 7-17 mm granules,

and a slump of 8 cm.

The concrete mix had excellent handling characteristies, the weight byvolume amount to 1.76 kg/litre, and the slump remained constant duringtransport, which indicates that the material did not absorb moisture.

The compressive strength of a cm cube, measured at the building site,was as high as 388 kgf/cm after only 7 days, and 415 kgf/cm after 14days. When, in a concrete of other wise the same composition, the abovepellets were replaced by expanded shale from a coal mine, thecompressive strength after 7 days was only 250 kgf/cm.

Since the concrete very rapidly assumes a high compressive strength,which later rises only very slowly, the material is very suitable foruse in prefabricated building elements of prestressed'concrete.

EXAMPLE Ill Shale coming from a coal washery and showing a loss onignition at 750C of 30.9 percent, a softening point of 1250C and ameltingpoint of 1,450C, was ground to a particle size below 0.2 mm. Theparticles were wetted and formed into granules of about 8 g of thefollowing compositions:

E. shale only F. 97.5 percent wt of shale and 5 percent wt of gypsum G.95 percent wt of shale and 5 percent wt of pumice,

in the size range 1.0 mm 0.1 mm

H. 92.5 percent wt of shale and 5 percent wt 0 pumice, ofthe size range1.0 mm 0.1 mm, and 2.5 percent wt of gypsum.

The pre-dried granules were heated in an oxidizing medium to about 750C,and kept at this temperature for one hour until the carbon originallycontained in them had completely disappeared. Thereupon, heating of thegranules in the oxidizing medium was continued at the rate of 30C perminute, up to 1,250C, which temperature was maintained for 10 minutes.After cooling, the volume by weight and the strength of the expandedgranules were determined.

Composition Weight by volume after Permissible load, kgf heating to I250"C H l.l8 125 chosen from the group consisting of non-expanding clay,weakly expanding clay, weathered slate, ground slate, weathered shale,ground shale and coal washery shale, having a relatively high softeningpoint of at least about 1,350C and a melting point of at least aboutl,600C;

B. an open pored, porous, inorganic, silicious material consistingessentially of white pumice having a particle size measuring up to 2 mm.in width and present in an amount equalling 10-20 percent by volume ofsaid clayey material; and

C. at least one material consisting essentially of gypsum which, whenheated to over 800C, but below the melting point of said open pored,porous inorganic material, yields gaseous components, present in anamount equalling up to 5 percent by weight of said clayey material;

granulating said mixture to produce granules having a range of sizes upto about 15 mm. across;

heating the granules up to a temperature over 1,350C at which saidclayey material is softened and said one material gives off gas,expanding the gas from uniformly distributed quanta of said open pored,porous, inorganic material as nuclei;

cooling the resultingly expanded granules to provide expanded granularaggregate having white or grey color on the walls of the expansioncavities formed by expanding said gas within said grgi ul e s.

2. Procesmrig a granular aggregate for use in light-weight constructionelements, comprising substantially uniformly mixing:

A. a non-expanding clayey material, chosen from the group consisting ofclay, weakly expanding clay, weathered slate, ground slate, weatheredshale, ground shale and coal washery shale having a relatively highsoftening point of at least about 1,350C and a melting point of at leastabout 1,600C;

B. an open pored, porous, inorganic, silicious material consistingessentially of white pumice having a particle size measuring up to 2 mm.in width and present in an amount equalling 10-20 percent by volume ofsaid clayey material; and

C. at least one material consisting essentially of gypsum which, whenheated to over 800C yields gaseous components, present in an amountequalling up to 5 percent by weight of said clayey material;

granulating said mixture to produce granules having a range of sizes upto about 15 mm. across;

heating the granules up to the softening temperature of said clayeymaterial and over the temperature at which said one material gives offgas, expanding the gas from uniformly distributed quanta of said openpored porous, inorganic material as nuclei; cooling the resultinglyexpanded granules to provide expanded granular aggregate having white orgrey color on the walls of the expansion cavities formed by expandingsaid gas within said granules.

2. Process for preparing a granular aggregate for use in light-weightconstruction elements, comprising substantially uniformly mixing: A. anon-expanding clayey material, chosen from the group consisting of clay,weakly expanding clay, weathered slate, ground slate, weathered shale,ground shale and coal washery shale having a relatively high softeningpoint of at least about 1,350*C and a melting point of at least about1,600*C; B. an open pored, porous, inorganic, silicious materialconsisting essentially of white pumice having a particle size measuringup to 2 mm. in width and present in an amount equalling 10-20 percent byvolume of said clayey material; and C. at least one material consistingessentially of gypsum which, when heated to over 800*C yields gaseouscomponents, present in an amount equalling up to 5 percent by weight ofsaid clayey material; granulating said mixture to produce granuleshaving a range of sizes up to about 15 mm. across; heating the granulesup to the softening temperature of said clayey material and over thetemperature at which said one material gives off gas, expanding the gasfrom uniformly distributed quanta of said open pored porous, inorganicmatErial as nuclei; cooling the resultingly expanded granules to provideexpanded granular aggregate having white or grey color on the walls ofthe expansion cavities formed by expanding said gas within saidgranules.